Monitoring battery status is an important issue for electronic devices, such as mobile phones, computers, laptops or the like. It is important for the user of a device that reliable fuel gauge indication is provided. Further, performing and monitoring charging and discharging procedures in an accurate and correct way facilitates for the user of the device. Still further, the lifetime of a battery may be shortened if the battery is not properly charged. Although a lot of different type of batteries may be used in such devices, a common type of rechargeable batteries used in electronic devices is Li-ion and Li-polymer batteries. One aspect of batteries, which applies basically for all current Li-ion and Li-polymer battery technologies, is that they tend to change dimension upon use. Attempts have been made to utilize change of dimension to monitor batteries.
An example of a known solution is presented in US2012/0286739 A1 disclosing a system and a method for estimating a state of a battery pack. The document discloses a method for estimating the state of a battery having multiple cells. In the known arrangement, strain gauges are coupled to battery binding bands that hold a plurality of cells of the battery together. The strain measured by the gauges is related to the electrical charge stored by the battery. The known method estimates battery state of charge during conditions when battery voltage changes little and the battery continues to accept charge. Since this solution uses multiple cells external band that groups the individual cells together is required for the measurements. Further, it is not possible to detect state-of-charge for an individual cell, only the sum state-of-charge for the group of cells. Thus, poor cell condition for an individual cell can be missed.
The known document relates mainly to LFP batteries. LFP refers to Li-ion cells using Lithium Iron Phosphate (LiFePO4) as active cathode material. LFP batteries has a very flat voltage profile and the flat discharge curve results in difficulty to use standard voltage-based methods for determining state-of-charge; typically high precision design is needed. In operation the cathode in a LFP battery basically gets depleted at full state-of-charge, thus voltage rises at this point. In e.g. in a LCO (Lithium Cobalt Oxide—LiCoO2) battery instead approximately half is used; if more Li is taken out the cathode becomes unstable. Therefore there is a need for reliable charge control and safety electronics.